Conventionally, as fixing devices used in image forming apparatuses of the electro-photographic method such as copying machines, printers, facsimiles, and multifunction peripherals provided with all these functions, fixing devices of heat roller type (which may be also called as heat fixing roller type) are widely adopted by from low speed equipments to high speed equipments as well as by from black-and-white equipments to color equipments. The fixing devices of heat roller type apply heat and pressure to recording medium with the toner image to be transferred thereon by gripping and conveying the recording sheet by a fixing nip portion formed by a fixing roller which is maintained in a predetermined range of temperature and a pressure roller that has an elastic layer, applies pressure and contacts to the fixing roller.
By the requirement of recent years for achieving colorization and speeding up, it is required to increase the nip width for fixing in order to effectively heat toner on the surface of the sheet. In order to increase the nip width, it is possible to consider increasing the diameters of the two rollers or increasing the dented (deformed) amount of the roller by increasing the contact force between the two rollers. However, if these measures are employed, problems such that the fixing device becomes big, and the reliability of the fixing device becomes low arise and the design flexibility becomes low.
In order to solve these problems, fixing devices of the belt nip type have been employed in these days that have an endless fixing belt which rotates driven by rollers and a press member which is fixed at a side of an inner peripheral surface of the fixing belt and presses the fixing belt towards the heat roller. Structurally, the width of the nip section of the fixing devices of the belt nip type can be wider than the width of the nip section of the fixing devices of the heat roller type.
Because the fixing devices of the belt nip type are configured that the fixing belt is pressingly contacted to the heat roller by the fixed sliding press member, running of the fixing belt is obstructed and sipping of image and wrinkle of sheet may be caused when the a friction between the fixing belt and the press member.
In order to solve such a problem, in the Unexamined Japanese Patent Application Publications 2002-148970, has been disclosed a fixing device that has a sliding sheet which is a sheet shaped member covering the press member for reducing a sliding friction with the inner surface of the fixing belt between, wherein a large undulation is formed on the inner surface of the sliding sheet, in order to prevent the slipping of image.
However, in the fixing device of the Unexamined Japanese Patent Application Publications 2002-148970, lubricant coated on the inner surface of the fixing belt and releasing agent coated on the surface of the heat roller are feared to seep from an edge to the inner surface of the sliding sheet and stick to the press pad covered by the sliding sheet. In such a case, the press pad swells and transforms by sticking oil such as the releasing agent and a problem is caused that the primary function does not work.
In fixing device of the Unexamined Japanese Patent Application Publication No. 2007-79183, in order to solve such a problem, a protruding section is provided outside of the sliding sheet so that the edge of the protruding section locates outside of the edge of the fixing belt of the fixing roller to prevent the releasing agent from seeping into the inner peripheral surface of the sliding sheet.
However a problem has occurred that the sliding sheet easily tears by wearing of the sliding sheet promoted by a friction between the sliding sheet and the fixing belt because length of the sliding sheet is made longer than the fixing belt by providing the protruding section.
Especially, in the sliding sheet having the large undulation on the surface, contacting with the fixing belt concentrates at the undulation section and therefore the problem that the tear easily occurs at the undulation section is caused.
By taking into considering of the above described problem, one object of the present invention is to provide a fixing device which prevents the sliding sheet from tearing by wearing of the sliding sheet which covers the press member for pressing the inner peripheral surface of the fixing belt.